A down flow type ice making machine is widely used as an ice making machine to produce a large quantity of ice blocks for its simple configuration and low ice-making cost. The down flow type ice making machine is configured in such a way that an evaporation pipe led out from a freezing apparatus is disposed at an ice making part provided upright vertically, and ice-making water is sprayed onto the ice making part cooled by a refrigerant which flows in the evaporation pipe in an ice-making operation, thereby producing ice blocks. The ice making machine is configured to have an ice-making water tank to store ice-making water, so that at the time of the ice-making operation, the ice-making water in the ice-making water tank is pumped out to the ice making part by an ice-making water pump, and ice-making water which has not been iced is collected into the ice-making water tank, and is then fed out toward the ice making part again. When the ice-making operation is shifted to a deicing operation after completion of ice making at the ice making part, the ice making machine causes a hot gas to flow through the evaporation pipe and sprays deicing water onto the back side of the ice making part to accelerate melting of icing surfaces of ice blocks with respect to the ice making part, and ice blocks separated from the ice making part are stored in an ice storage chamber. The deicing water is collected into the ice-making water tank, and is used as ice-making water in the next ice-making operation.
In the ice making machine, in the ice-making operation, a portion of the ice-making water excluding an impurity, such as calcium, contained therein is iced at the ice making part, and the impurity is collected together with uniced water into the ice-making water tank. That is, the impurity in the ice-making water stored in the ice-making water tank is gradually condensed by the repetitive ice-making operation. This brings about problems, such as impurity-originated clogging of the ice-making water pump, piping or the like to supply ice-making water to the ice making part, and reduction of the ice-making efficiency originated from adhesion of the impurity to the ice making part.
In this respect, there has been proposed an ice making machine which performs a water-discharge operation of discharging ice-making water remaining in an ice-making water tank at the ice-making completion water level outside after an ice-making operation is completed (see Patent Document 1). The ice making machine disclosed in Patent Document 1 has water discharge means that includes an overflow pipe which defines the maximum water level of ice-making water remaining in the ice-making water tank, a water discharge pipe connecting an ice-making water pump to pump out the ice-making water from the ice-making water tank to an ice making part to the overflow pipe, and a valve to open/close the passage of the water discharge pipe. In the water-discharge operation, the ice making machine of Patent Document 1 rotates the ice-making water pump in the reverse direction to the rotation in the ice-making operation, and releases the valve to discharge the ice-making water outside via the water discharge pipe and the overflow pipe. In the ice making machine, counting a predetermined discharge time is started at the same time as the driving of the pump and the releasing of the valve, and when the discharge time passes, the pump is stopped and the valve is closed, terminating the water-discharge operation. After the water-discharge operation is completed, the ice making machine starts a deicing operation.    PATENT DOCUMENT 1: Japanese Patent Application Laid-Open No. Hei 5-45033